1. Field of the Invention
The present invention relates to a method for detecting foreign bodies in a medium with a radar and which includes emitting a radar wave with a transceiver into the medium, detecting the signal reflected in the medium, pre-processing the reflected signal, and subjecting the pre-processed signal to evaluation and assessment. The present invention also relates to the use of the inventive method in an electromagnetic or position sensor for detecting foreign bodies in a medium.
2. Description of the Prior Art
Radars, in particular ground radars, are used in geodesic works for a long time. For the first time, the use of a radar principle for detecting metal, plastic parts and other foreign bodies in a wall was described in the publication of D. J. Daniels, Surface-Penetrating Radar, IEE-Press, London, 1996 Daniels. At present, all available on the market radars are based on so-called image bending method which supply to a user xe2x80x9cimagesxe2x80x9d of the obtained measurement data. The conventional radar apparatuses are expensive, relatively large, and are hardly usable under xe2x80x9csmokedxe2x80x9d everyday conditions prevailing at a constructional site. In order to obtain the image data, a manually or mechanically controlled scanning process, i.e., movement of the antenna unit is necessary for collecting a large amount of data. U.S. Pat. Nos. 5,541,605 and 5,543,799 disclose comparatively small radars for detecting foreign bodies in walls. However, the data obtained during the scanning process with these radars are not use or are not used in optimal manner. U.S. Pat. No. 5,512,834 discloses a relatively small electromagnetic sensor for detecting presence of metallic and non-metallic objects in the walls. However, the signal evaluation is effected only with respect to certain predetermined level parameters.
The functional principle of a impulse radar which, together with a stepped frequency radar, is of a primary interest here, can be explained with reference to FIG. 3. With a clock high-frequency generator 1, a very short pulse having a length or duration less than 1 ns is generated. This pulse is emitted by an antenna 2 as an electromagnetic wave into an examined medium (e.g., concrete). At the locations where the permittivity changes, e.g., in transition areas concrete/reinforcing metal, concrete/plastic pipe, the electromagnetic wave(s) is (are) reflected. This reflection is detected by the second antenna 3. The reflected signal is amplified by a high-frequency amplifier 4 which is controlled by a scanning control unit 7 in a time-dependent manner. The amplified signal is communicated to a band-limiting unit 8 and then to a scan H-circuit 5. Finally, the signal is digitalized by an A/D converter 6 and communicated to signal processing means. The entire signal pre-processing equipment or at least the transmitting and receiving antennas 2 and 3 are located in a common housing which can be easily handled. The antennas 2 and 3 are displaced across the examined medium, and the propagation, together with the amplitude of the reflection signal are plotted graphically dependent on a scanning position (see J. Hugenschmid, xe2x80x9cGeoradar in Strassenbau und Unterhalt (Geodsic Radar in Road Construction and Maintenance), xe2x80x9can extract from EMPA-Report (113/10), No. FE160,617, pp. 9/93-11/93). A technician is only in a position to make a conclusion, based on the obtained diagram or images, whether foreign bodies are present in the examined medium. For further details, please refer to Daniels.
The main difficulty of using such radar apparatuses consists in that the scanning process need be continuously conducted over a large surface area in order to find out at which points the reflection signal are generated due to the presence of foreign bodies. For the interpretation of the obtained xe2x80x9cimagesxe2x80x9d, experience and technical knowledge are necessary. Besides, the conventional radars, as it has already been mentioned above, are large and expensive.
Accordingly an object of the present invention is to provide an improved method for detecting positions of metal, plastic and/or other foreign bodies enclosed in media such as concrete, brick, gypsum, plaster, wood, and the like and in surrounding media such as air, gas or liquid.
Another object of the present invention is to provide detecting apparatus based on the improved detecting method.
A further object of the present invention is to provide an improved method for detecting positions of metal, plastic and/or other foreign bodies enclosed in media such as concrete, brick, gypsum, plaster, wood, and the like and in surrounding media such as air, gas or liquid, with which a continuous displacement of the antenna unit across the examined surface becomes unnecessary.
These and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a method for detecting foreign bodies which includes determining a characteristic of a foreign body at a stationary position of the transceiver with an algorithm which is based on a pre-processed, digitalized receivable signal, which characteristic permits to recognize differences between a shape of the reflected radar signal and stored comparison values, and conducting a comparison assessment of the shape differences of the reflected signal against predetermined threshold values, whereby a position of the foreign body is determined.
The present invention is based on a concept, in particular when an impulse radar is used, on exciting the transmitting antenna with very short pulses to obtain an electromagnetic wave with different amplitudes and frequencies which can be emitted in different directions. When a reflection from a foreign body takes place at a predetermined angle with respect to the transceiver antenna, the reflection signal has a shape different from a shape of a reflection signal obtained directly beneath the antenna. The method of signal processing according to the present invention, which will be describe in detail further below, permits to distinguish between these two signals due to their different amplitudes and/or frequencies.
For achieving even better signal/noise ratios, advantageously, a stepped-frequency radar can be used.